1. Field of the Invention
This invention relates to a fluid pressure cylinder, and more particularly to a rod-less cylinder of a slit type.
2. Description of the Related Art
In recent years, rod-less cylinders have been used which do not need any piston rods as an alternative to cylinders of the rod type in order to save a space. A cylinder of the type described above comprises, as shown in FIG. 7, an elastic sealing member 103 for sealing a slit 102 formed in the axial direction of a cylinder 101, and an outer cover 104, whereby the movement of a piston 106 is conducted to a mounting body 107 through a connecting member 105 projecting outwardly through the slit 102.
In a cylinder of the type described above, in order to prevent the sealing member 103 from becoming separated from the slit 102 even if no internal pressure exists in the cylinder 101, means for retaining the sealing member 103 in the slit 102 is required. Furthermore, in order to avoid the internal pressure in the cylinder 101 affecting the sealing member 103, the width of the slit 102 is preferably reduced.
As a means for retaining such a sealing member in the slit, a structure is disclosed in U.S. Pat. No. 4,664,020 in which they are connected by way of inserting a projecting disposed in the upper surface of the sealing member into a space between a pair of leg members projecting downwardly from the outer cover.
However, in a cylinder of the type described above, the connected portion of the leg members of the outer cover with the projection of the sealing member needs to be accommodated in the slit, and the width of the slit therefore needs to be rather large. As a result of this, a large sealing member is needed for the purpose of bearing the internal pressure of the cylinder. This leads to the problem that the size of the cylinder cannot be reduced.
Furthermore, since the sealing member is hung from the leg portion of the outer cover for the purpose of support, it can easily slip off in the direction of the width of the slit in this slit, causing the sealing lip thereof to be separated from the inner surface of the cylinder.
A rod-less cylinder is also disclosed in U.S. Pat. No. 4,545,290 which comprises a sealing member constituted by a retaining member secured in a slot and a sealing portion for sealing the inner surface of the cylinder.
However, when the sealing member described above is subjected to the internal pressure of the cylinder, the sealing member is raised in the outward direction of the cylinder, causing the sealed portion to be bent in the direction of the inside of the cylinder relative to the two inner ends of the slot to act as a fulcrum. If the sealed portion is bent, air in the cylinder can leak through the sealed portion between the sealing lip surface and the inner surface of the cylinder. As a result of this, the airtightness of the cylinder cannot be maintained, and the efficiency with which the piston is driven thus deteriorates.
Furthermore, in the disclosed U.S. Pat. No. 4,545,290, since the inner surface of the cylinder is formed with an annular shape, the curvature of the sealing lip surface of the sealed portion should conform to that of the inner surface of the cylinder. Therefore, the work involved in manufacturing the sealing member becomes complicated. Furthermore, since the inner surface of the cylinder is formed in a curved surface while the top surface of the piston has a flat surface, a gap is created between the inner surface of the cylinder and the top surface of the piston at the two ends of the sealing lip. Consequently, a problem of air leakage at this gap arises.